The present invention relates to wall thermostats of the type employed for controlling the service cycling of a heating plant for a building. In particular, the invention relates to thermostats of the type for controlling on-off cycling of electrical heating devices, as for example, resistance heaters.
In providing controls for buildings having electrical heating systems, it is desirable to provide a control or thermostat capable of switching load current at line voltage for cycling the heating system.
A controller or thermostat which is capable of performing a load current switching function at line voltage, eliminates the need for costly relay, transformer and the associated extra wiring required in the building for central control.
In addition, the use of a line voltage thermostat facilitates heat regulation by permitting individual thermostat control of separate heaters for each room or zone of the building.
In recent times there has developed an interest in thermostats or controllers for heating systems which can perform an automatic function at a preselected desired time of day for cycling the heating system to operate in a manner so as to maintain the building at a desired temperature less than the normal temperature when the building is occupied. Thermostats or controllers which perform this function are known as "setback" thermostats and employ a timing or clock mechanism which may be set by the user to operate in a setback mode during the desired times when the building will be unoccupied.
Setback thermostats are popular in newly constructed commercial and residential buildings; and, provision is generally made for such type thermostats when the heating system controls are installed and wired into the newly constructed building. However, where it is desired to provide such a setback function for an existing thermostat in an older buildings, problems have arisen. Typically, thermostats in older buildings are wired onto a single width outlet box. This was generally because the control operated on low e.g. 23 volts and was simple, consisting only of a bi-metal actuated switching mechanism and did not employ any clock mechanism requiring additional support wiring space in the outlet box.
Where it has been desired to add or retrofit a setback type thermostat for an existing line voltage thermostat installation, it has been found difficult to provide a way of mounting the thermostat and clocking mechanism onto a single width electrical outlet box as is commonly found in older line voltage thermostat installations.
In buildings having plaster or plaster board walls, building and fire safety codes require that any line voltage wiring extending on the external surface of the wall be encased in metal and have a metal protector plate between the wiring and the surface of the plaster or plaster board. Further, it is usually required by building and fire safety codes to have all line voltage switching mechanisms on the surface of the wall completely encased in metal such that in the event of a short in the switching mechanism, molten metal will not spatter on any material other than metal. Further, it is required by building and safety codes that all line voltage wiring on the surface of a wall be permanently encased in such a manner that users cannot readily access the wiring by simply removing the outer bezel or casing for the thermostat.
Thus, it has long been desired to find a way or means of providing a line voltage setback thermostat that can be retrofitted to existing thermostat installations on a single width electrical outlet box. It has further been desired to provide a line voltage setback thermostat which may be retrofitted to a single width flush mounted or released wall outlet box and one which minimizes the hazards of fire in the event of electrical shorting and also electric shock in the event of removal of the bezel or cover.